1. Field of the Invention
The present invention pertains to new and useful improvements in reducing the fluid loss of cement slurries. Cement formulations of the invention also exhibit improved interfacial sealing between the hardened cement and pipe or borehole wall.
2. Description of Prior Art
Subsequent to drilling a borehole of an oil or gas well, casing is run into the well and a cement slurry is placed in the annulus between the outside of the casing and the borehole wall. Typically, the cement slurry is placed in the annulus using a successive fluid displacement technique. In this technique, the cement is first pumped into the casing. Another fluid, often drilling fluid or water, is pumped behind the cement to displace it out the bottom of the casing and up into the annulus between the casing and borehole wall. The cement slurry is usually raised to a point above the uppermost section of the formations to be isolated and may be raised into the annulus between the casing string being cemented and a previously cemented casing.
Differential pressure is required to place the cement in the annulus. A differential pressure typically exists between the fluid column in the annulus and the pore pressures of the exposed formations. The hydrostatic pressure in the annulus immediately after the cement is placed is usually higher than the pore pressure of exposed formations in order to prevent flow of formation fluids into the annulus.
Since cement slurries are suspensions of solids in water and the exposed formations of the wellbore are permeable, filtration can occur. In this case, fluid from the cement slurry can be lost to the exposed permeable formations through filtration resulting from the pressure differential between the annulus and formations. To prevent this, fluid loss control agents are added to the cement slurry. Typically, these fluid loss agents are polymeric materials. A considerable variety of polymer classes exist which will function to reduce the fluid loss of cement and others are being developed.
Many types of polymeric fluid loss control agents increase the viscosity of the cement slurry. Surface active agents such as naphthalene sulfonate formaldehyde condensates, phenol sulfonate formaldehyde condensates, and lignosulfonates are commonly added to cement slurries to disperse the cement particles and fluid loss additive, and reduce the slurry viscosity. In many cases, the dispersing and viscosity reducing surface active agent was preblended into the fluid loss additive mixture. Examples of this art are contained in U.S. Pat. Nos. 3,132,693, 4,258,709, 4,433,731, 4,687,516, 4,569,395, and 4,742,094.
Some of the surfactants of the type used in the present invention have been employed in the preparation of liquid fluid loss additives which may be added to the cement slurry (U.S. Pat. No. 4,687,516). Such surfactants are used for a different purpose than in the present invention, namely to water wet a polymer upon contact with water in the preparation of the cement slurry. An organic dispersing agent of the type previously described is also present in this prior art.
It will be shown by data presented in the detailed description of the present invention hereinafter that a certain type of surfactant alone can reduce the fluid loss of the cement slurry by interacting with the polymer fluid loss agent in the slurry.